1. Field of the Invention
This invention relates to a vibration wave driven motor for use in a camera, business machine or the like, and particularly to the vibration member thereof.
2. Related Background Art
A motor has already been proposed in which at least two groups of piezo-electric elements spatially deviated from each other are provided on a resilient member formed of a metal or like material and electrical signals differing in phase from each other are applied to the respective groups of piezo-electric elements, whereby a travelling wave is generated on the resilient member and a moving member which is in pressure contact with the resilient member is driven by the travelling wave.
Also, such motor has a feature that its torque during low speeds is relatively great and therefore, has been applied to visual instruments in recent years.
The shape of the vibration member of such vibration wave driven motor, i.e., the vibration member comprising piezo-electric elements and a resilient member, is generally classified into a circular shape and a non-circular shape, e.g. an elliptical shape, and in a vibration member of a non-circular shape, there has arisen a problem that torsion is produced in vibration and reduces the efficiency of the motor and noise is produced from the portion of contact between the vibration member and the moving member.
This point will further be described with reference to FIG. 8 of the accompanying drawings.
The reference numeral 1 designate the aforedescribed resilient member. The resilient member 1 used in the conventional vibration wave driven motor is of a predetermined cross-sectional shape as shown in FIG. 8. The reference numeral 2 denotes piezo-electrice elements as electro-mechanical energy conversion elements. The polarization and arrangement of the piezo-electric elements 2 and the manner of application of electrical signals to the piezo-electric elements 2 are as in the past. In the case of FIG. 8, the piezo-electric elements 2 are vibrated in a vertical direction to thereby generate a travelling wave on the elliptical resilient member 1, and the straight portions of the resilient member 1 are used to feed a sheet (not shown) rectilinearly. Slits are provided in the resilient member 1 to increase the sheet feeding speed.
However, the circumferential length of the travelling wave generated on the resilient member 1 differs between the inner diameter and the outer diameter of the arcuate portions of the elliptical resilient member and therefore, other torsion component than required vibration has been produced.
FIGS. 9 and 10 of the accompanying drawings illustrate the torsion of the vibration produced on the resilient member 1, and represent the contour lines of displacement in a direction perpendicular to the piezo-electric elements 2 caused in the resilient member 1 by the vibration of the group A (not shown) and the group B (not shown) of the piezo-electric elements which have been found by an eigenvalue analysis using the finite element method. As regards the amount of displacement, maximum displacement is normalized as "1".
As is clearly shown in an area on the resilient member which is indicated, for example, by dotted line A in FIG. 10, in the conventional vibration wave driven motor, the amplitude of vibration has been high on the outer peripheral side (the right side) of the elliptical resilient member 1, and as regards also the displacement of vibration generated on the resilient member by another group of piezo electric element shown in FIG. 9, as is clearly shown in an area indicated by dotted line a, the amplitude of the vibration has been high on the outer peripheral side of the resilient member 1 and low on the inner peripheral side thereof.
That is, the vibration on the resilient member of the conventional non-circular vibration wave driven motor has been in a state in which displacement in the same area is not equal, in other words, in a state in which the vibration on the resilient member is distorted.
This has led to the disadvantage that the efficiency of the motor is low as previously described and sound is produced between the vibration member and a member moving relative to the vibration member, for example, a rotor.